Genes encoding proteins with readily detectable activities, referred to as reporter proteins, are routinely used in biological assays to study a variety of biological events. These biological events include gene expression, gene transfer and the intracellular movement of molecules. Genes encoding reporter proteins which are capable of generating light emission such as luciferases, in particular, have been incorporated into sensitive, noninvasive biological assays which are simple to perform. The most widely used luciferases used in assays are encoded by genes from the biolumninescent Vibrio bacteria, the jellyfish Aequoria victoria, and the firefly Photinus pyralis. 
Further, genes encoding reporter proteins that are secreted offer distinct advantages when they are incorporated into assays because they permit the monitoring of gene expression over time without destroying the cells or tissues which are being studied. Secreted alkaline phosphatase (SEAP) is an example of a light-emitting, secreted reporter protein whose encoding gene is useful in mammalian cell assays (see, for example, Yang, T. T. et al. xe2x80x9cQuantification of gene expression with a secreted alkaline phosphatase reporter system.xe2x80x9d Biotechniques, 23: 1110-1114 (1997)). Other light-emitting, secreted reporter proteins are known, including the luciferase from the marine ostracod Vargula hilgendorfii, or from the decapod shrimp Oplophorus, but their use is restricted because either the luciferin substrates for their secreted proteins are not commercially available or their encoding genes have not been cloned.
Additionally, dual reporter assay systems are often used for studies of gene expression in transiently transfected cells. In these systems, one reporter gene is fused to a DNA promoter element of interest, while the other reporter gene is fused to a constitutive promoter. Measurements obtained from the expression of the latter are then taken for normalization between experiments. A good dual reporter system should offer assays that are both sensitive and simple to perform.
Therefore, it would be useful to have other genes encoding secreted reporter proteins which could be incorporated into biological assays and which have commercially available substrates for the secreted reporter proteins they encode. Further, it would be useful to have vectors containing these genes to prepare kits for performing the assays. Additionally, it would be useful to have a dual reporter system that is both sensitive and simple to perform.
In one embodiment, there is provided a polynucleotide encoding a secreted Renilla luciferase. According to another embodiment of the present invention, there is provided a secreted Renilla luciferase.
According to still another embodiment of the present invention, there is provided a method of performing a biological assay. The method comprises providing a polynucleotide encoding a secreted Renilla luciferase. The method can additionally comprise transfecting a host cell, such as a mammalian cell, with the polynucleotide. The method can also comprise detecting light emission from the Renilla luciferase coded by the polynucleotide that has been secreted in the culture media in which the host cell is growing. The method can further comprise transfecting the host cell with a second polynucleotide encoding a second light emitting protein such as seap. The method can also comprise detecting light emission from the second light emitting protein coded by the second polynucleotide that has been secreted in the culture media.
According to another embodiment of the present invention, there is provided a plasmid or a vector containing a polynucleotide according to the present invention, as well as a host cell, such as a mammalian cell, transfected with a polynucleotide according to the present invention. The present invention can also include a kit for performing a biological assay, comprising a polynucleotide according to the present invention. The kit can include a second polynucleotide, such as seap.
According to another embodiment of the present invention, there is provided a stable mammalian packaging cell line which produces retroviruses carrying a polynucleotide encoding a secreted Renilla luciferase.